Ubuntu Xenial fails to mount software RAID-1 if fstab records for /dev/mdX are present, ignores mdadm.conf

Question

I'm migrating my old Arch Linux system to Ubuntu Xenial and I ran into a problem with my RAID devices.

The RAID-1 devices are described as following:

iliv@sega:~$ grep ARRAY /etc/mdadm/mdadm.conf 
ARRAY /dev/md0 metadata=1.2 name=sega:0 UUID=c4cf4a52:6daa94c8:6d88a2fa:8f604199
ARRAY /dev/md1 metadata=1.2 name=sega:1 UUID=f7f0d240:f9fb5369:bdfe8219:325d8184

There are corresponding records in /etc/fstab:

/dev/md0        /mnt/RAIDVault-BACKUP   ext4    defaults 0 0
/dev/md1        /mnt/RAIDVault  ext4    defaults 0 0

What happens is that if these two lines are in /etc/fstab when Xenial is booting it apparently ignores mdadm.conf, assigns my devices /dev/md126 and /dev/md127 device names, fails to mount them obviously because there's nothing in /etc/fstab explaining what to do about /dev/md126 and /dev/md127 and eventually drops me into emergency shell.

In the emergency shell I can see that my devices are in "auto-read-only" mode, assembled and healthy.

Now, if I comment out RAID records in /etc/fstab the system boots up just fine, again assigning /dev/md126 and /dev/md127 to RAID devices. Most interestingly, if immediately after the system has started up I disconnect and reconnect my RAID devices the get assembled as /dev/md0 and /dev/md1 as they should have and always been. I can then uncomment /etc/fstab entries and mount them as usual and everything is fine.

So, what is going on here and how can I get "what part of Xenial is this??" to honor mdadm.conf which it seemingly does not?

Answer 1

My understanding is that the reason this is happening is because my software RAID devices were not originally created on this system. As a result initramfs image knows nothing about them and apparently attempts to assign some kind of default device names, /dev/md126 and /dev/md127.

To fix this problem all I had to do was run:

sudo update-initramfs -k all -u